1. Field of the Invention
The present invention relates to disk drives. In particular, the present invention relates to a disk drive that adjusts the write clock frequency to compensate for eccentricity in disk rotation.
2. Description of the Prior Art
A disk drive typically comprises one or more disks rotated by a spindle motor while heads are actuated radially over the disk surfaces. Each disk surface comprises a number of radially spaced, concentric tracks, where each track is divided into a number of data sectors. A number of embedded servo sectors forming servo wedges are also written on each disk surface, which facilitate seeking the head and maintaining the head over the centerline of a target track during read and write operations.
The disks are rotated at a constant angular velocity (CAV) while varying the data rate from an inner diameter zone to an outer diameter zone to maximize the recording density by maximizing the usable disk space at the outer diameter tracks. That is, the write clock frequency is adjusted relative to the radial location of the head so as to maintain a substantially constant linear bit density from the inner to outer diameter tracks correlative with an increasing linear velocity from the inner to outer diameter tracks. Ideally, the linear bit density for each zone on the disk is selected very near the maximum, that is, at a density where interference between adjacent bit cells is tolerable. However, eccentricity in the disk rotating will induce a sinusoidal disturbance in the linear velocity of the tracks relative to the head leading to a sinusoidal disturbance in the linear bit density. Eccentricities may occur, for example, if a media writer is used to servo write the disk before installing the disk into the disk drive, if the disk “slips” after using the head internal to the disk drive to servo write the disk, or if the disk slips after writing user data to the data sectors. Thus, prior art disk drives decrease the linear bit density selected for each zone on the disk by some predetermined margin to account for the worst case disturbance in the linear velocity, which leads to an undesirable decrease in storage capacity.
There is, therefore, a need to increase the storage capacity of disk drives by better optimizing the linear bit density from the inner to outer diameter zones.